The present invention relates to the production of fissile fuel for fission reactors, and in particular to the breeding of such fuels using the neutrons generated by a fusion reactor
Fertile materials such as thorium 232 (.sup.232 TH) and uranium 238 (.sup.238 U) may be bombarded by neutrons to produce fissile materials such as uranium 233 (.sup.233 U) and plutonium 239 (.sup.239 Pu), respectively. A "fissile" material is one which is fissionable by slow neutron capture, and a "fertile" material is one which can be rendered fissile by neutron absorption. The source of the neutrons may be a fusion reactor. The fusion breeder concept is attractive because the subsequent burning of a bred fissile atom releases 200 MeV as compared to the 20 MeV typically produced by a single fusion reaction.
Generally, the fusion neutron enriched fuel must be chemically reprocessed so that the concentration of fissile fuel is sufficient for fabrication of the fuel elements (blocks, control rods, pins, etc) which are to be inserted into a fission reactor. Reprocessing is subject to certain dangers which are the focus of resistance to the development of nuclear reactors. Not the least objectionable aspect of reprocessing are the opportunities provided for diversion of materials which could be used to manufacture a nuclear explosive device. Reprocessing is also expensive, adding up to $50 per gram or more to the cost of the bred fuel.
In part to circumvent such problems, it has been proposed to bypass reprocessing by breeding fuel within assembled fuel elements, pins and rods inserted into the blanket of a fusion reactor. The problems introduced by this approach include thermal and radiation damage to structural material of the fuel element Additionally, since the neutron flux is not generally uniform throughout the fusion blanket, the eventual fuel elements contain variably enriched fuel.
The relatively long residence time required adversely affects the materials and economics involved. For example, in one approach the calculated residence time is on the order of 2.6 years. Furthermore, approximately 11-12% of the .sup.233 U bred in the blanket is burned up before 4% enrichment is achieved. [R. W. Conn, S. I. Abdel-Khalik, G. A. Moses, G. L. Kulcinski, E. Larsen, C. W. Maynard, M. M. H. Ragheb, I. N. Sviatoslavsky, W. F. Vogelsang, W. G. Wolfer, M. Ortman, R. Watson and M. Z. Youssef, "Fusion-Fission Hybrid Design with Analysis of Direct Enrichment and Non-Proliferation Features (The Solase-H Study)", Nuclear Engineering and Design, 63 (1981) pp. 357-374.]
Accordingly, it is an object of the present invention to provide a method for enriching fertile material using a fusion reactor, which method does not require chemical reprocessing, provides for uniformity of enrichment, and does not damage the structural components of the fuel elements. It is also an object of the invention to provide for a shortened residence time of the fission fuel in the fusion blanket.